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This chapter is from the book

1.4 Sculpting the PDN Impedance Profile

The goal in PDN design is to engineer an acceptable impedance profile from DC to the highest frequency component of any power rail currents. All the elements of the PDN should be engineered together to sculpt the impedance profile of the entire ecology. Although many elements interact, assigning some features of the PDN impedance profile to specific features in the PDN design is possible.

Figure 1.8 shows a simplified schematic of the entire PDN ecology. This includes the on-die capacitance, the possibility of on-package capacitors, the package lead inductance, the circuit board vias, the power and ground planes in the circuit board, decoupling capacitor, bulk capacitors, and VRM.

Figure 1.8

Figure 1.8 Top: Simplified schematic of the PDN ecology showing the major elements. Bottom: Resulting impedance profile identifying how specific design features contribute to specific impedance features. On the horizontal scale “x” is MHz.

Isolating functions of some PDN elements enables us to optimize parts of the PDN independent of the others, as long as we always pay attention to the interfaces where the impedance of one element interacts with the impedance of another. This is why so much of PDN design is about the interfaces between the parts.

In the journey ahead, we explore each of these elements that make up the PDN and how they interact to result in a robust and cost-effective PDN design. Ultimately, the power integrity engineer is responsible for finding an acceptable balance between cost, risk, performance, and schedule. The more we know about the details of the specific PDN elements, the more quickly we can reach an acceptable solution.

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